Chapter 30: Amino Acid Degradation and the Urea Cycle

Question 1

What is a ketogenic amino acid?

Answer 1

Amino acid carbon skeleton converted to acetyl CoA or acetoacetate

Question 2

What is a glucogenic amino acid?

Answer 2

• Carbon skeletons converted to pyruvate, alpha-ketoglutarate, succinyl-CoA, fumarate or oxaloacetate
• Amino acids finally converted to glucose through gluconeogenesis

Question 3

Amino acids are first used to build new _____. Those not needed for that are catabolized to make use of their _____.

Answer 3

• New proteins
• Make use of their nitrogen and carbon skeletons

Question 4

Amino acid degradation primarily occurs in the _____.

Answer 4

Liver

Question 5

First step is often removal of the alpha-amino group to generate _____.

Answer 5

Ammonia

Question 6

After generation of ammonia, the carbon chains are altered for entry into _____.

Answer 6

Central pathways of carbon metabolism

Question 7

Amino acids are made of _____ and _____.

Answer 7

Amine group and carboxylic group

Question 8

What are the 3 key reactions of disposal of amino acids nitrogen?

Answer 8

1. Transamination reactions
2. Deamination reactions
3. Glutamine Synthesis

Question 9

The exchange of the NH2 group with =O group on another molecule is considered to be…

Answer 9

Transamination

Question 10

The first key reaction of disposal of amino groups results in the following reaction:

Answer 10

Amino acid + Keto acid –> Keto acid + Amino acid

• Transamination
• One becomes the other

Question 11

What catalyzes the transfer of the alpha-amino group of aspartate to alpha-ketoglutarate?

Answer 11

Aspartate transaminase

Question 12

There are only a few keto acid acceptors. What is the most used?

Answer 12

Alpha-ketoglutarate

Question 13

The second key reaction of disposal of amino groups has two parts:

Answer 13

Deamination

• Oxidative deamination - aerobic conditions
  
  • Catalyzed by glutamate dehydrogenase
  • Important in liver where it releases ammonia for urea synthesis
• Hydrolytic deamination
  
  • Catalyzed by glutaminase and asparaginase

Question 14

The third key reaction of disposal amino groups:

Answer 14

Glutamine Synthetase

NH3 –> Glutamate –> Glutamine

Question 15

Used as a carrier of ammonia in most tissue

Answer 15

Glutamine

Question 16

Used as a carrier in muscles

Answer 16

Alanine

Question 17

What is the 1st step in amino acid degradation?

Answer 17

Nitrogen removal

Question 18

Some amino acids can be _____ deaminated.

Answer 18

Directly

Question 19

Removal of ammonium from most amino acids involves two steps:

Answer 19

1. Aminotransferases (transaminases) transfer amino groups from an amino acid to alpha-ketoglutarate to generate glutamate
2. Glutamate dehydrogenase, a mitochondrial enzyme, releases NH4+ in the oxidative deamination of glutamate

Question 20

Peripheral tissues transport nitrogen to the _____.

Answer 20

Liver

Question 21

Nitrogen can also be transported as _____ formed from glutamate by glutamine synthetase

Answer 21

Glutamine

Question 22

Ammonia is toxic and needs…

Answer 22

To be excreted

Question 23

What happens to excess ammonia?

Answer 23

Urea Cycle

Question 24

What three molecules come into the urea cycle?

Answer 24

Ammonia and CO2 and Aspartate

Question 25

Functions as detoxification of ammonia (prevents hyperammonemia)

Answer 25

Urea Cycle

Question 26

Overall Urea Cycle occurs in the \_\_\_\_\_

Answer 26

Liver

Question 27

Urea Cycle takes CO2, NH3 and aspartate + the cost of _____ to produce...

Answer 27

Cost of 3 ATP To produce Urea and Fumarate

Question 28

Glutamate Dehydrogenase reaction

Answer 28

Glutamate + NADP+ --\> alpha-ketoglutarate + NH3 + NADPH ## Footnote **Generation of NH3**

Question 29

CPS Reaction:

Answer 29

NH3 + HCO3- + 2 ATP is converted to Carbamyl Phosphate by Carbamyl Phosphate Synthetase

Question 30

CPS I (Mitochondria) uses _____ which is important for \_\_\_\_\_.

Answer 30

Uses NH3 which is important for urea cycle

Question 31

CPS II (Cytosol) uses _____ which is important for \_\_\_\_\_.

Answer 31

Uses glutamine which is important for pyrimidine biosynthesis

Question 32

Bicarbonate is activated by _____ to form \_\_\_\_\_.

Answer 32

Activated by ATP to form carbonyl-P

Question 33

Carbonyl-P is converted to \_\_\_\_\_.

Answer 33

Carbamate

Question 34

Carbamate is phosphorylated by \_\_\_\_\_. Carbamate converts to \_\_\_\_\_.

Answer 34

By ATP Converted to carbomoyl phosphate

Question 35

Urea Cycle: Carbamoyl P + _____ --\> _____ by \_\_\_\_\_.

Answer 35

Ornithine --\> Citrulline by Ornithine transcarbamylase

Question 36

Ornithine + Citrulline are considered _____ amino acids.

Answer 36

* Non-standard (not present in proteins) * Carry ammonium group from cytosol to mitochondria or vice versa

Question 37

What gets added from the cytosol to citrulline?

Answer 37

Aspartate

Question 38

Citrulline + Aspartate --\> _____ by \_\_\_\_\_.

Answer 38

Argininosuccinate by Argininosuccinate synthetase

Question 39

Elimination reaction leads to _____ and _____ (cytosol).

Answer 39

Arginine and fumarate

Question 40

Argininosuccinate --\> _____ by \_\_\_\_\_.

Answer 40

Arginine by Argininosuccinase (cytosol)

Question 41

In the cytosol, arginine is converted to _____ by _____ which is transported into the \_\_\_\_\_.

Answer 41

Converted to Ornithine by Arginase (cytosol), which is transported into the mitochondria

Question 42

Fumarate can be converted into _____ by the _____ and then into _____ by the \_\_\_\_\_.

Answer 42

Can be converted into **oxaloacetate** by _the citric acid cycle_ and then into glucose by gluconeogenic pathway.

Question 43

The urea cycle is generally controlled by \_\_\_\_\_

Answer 43

Substrate availability

Question 44

The more ammonia, the higher the rate of \_\_\_\_\_.

Answer 44

Urea formation

Question 45

Defects in any of the urea cycle enzymes results in elevated levels of _____ in the blood, which causes nervous system malfunction and can be lethal.

Answer 45

NH4+

Question 46

Excessive alcohol consumption causes...

Answer 46

Liver cirrhosis

Question 47

* Defects in catabolism of branched-chain amino acids * Inability to degrade branched amino acids

Answer 47

Maple Syrup Urine Disease

Question 48

Which amino acids can be directly deaminated to produce NH4+? A. Serine and threonine B. Proline and valine C. Phenylalanine and tryptophan D. Glycine and alanine E. No amino acids can be directly deaminated

Answer 48

Serine and Threonine

Question 49

Tryptophan is degraded to acetacetyl CoA and pyruvate, making it:

Answer 49

Both a ketogenic and glucogenic amino acid

Question 50

Phenylketonuria is caused by a defect in: A. Phenylalanine reductase B. Succinyl CoA synthetase C. Citrulline dehydrogenase D. Phenylalanine hydroxylase E. Tyrosine transaminase

Answer 50

Phenylalanine hydroxylase

Question 51

Essential AAs generally..

Answer 51

Generally require higher amounts of energy for their synthesis than non-essential amino acids

Question 52

Which serves a catalytic role in the urea cycle? A. Carbamoyl phosphate B. Aspartate C. Ornithine D. Glutamate E. ATP

Answer 52

Ornithine

Question 53

People with Maple Syrup Disease accumulate _____________ in their blood. A. Leucine, isoleucine and valine B. Homogentistic acid C. Phenylalanine D. Alpha-keto acids E. Urea

Answer 53

Alpha-keto acids

Question 54

Tyrosine can be broken down to fumarate and acetoacetate. Tyrosine is: A. Solely a ketogenic amino acid B. Solely a glucogenic amino acid C. Both a ketogenic and a glucogenic amino acid D. Neither a ketogenic nor a glucogenic amino acid E. It is impossible to tell from this information

Answer 54

Both a ketogenic and a glucogenic amino acid

Question 55

In mammals, nitrogen present in amino acids:

Answer 55

Comes primarily from dietary protein

Question 56

Lysine is degraded through a series of steps to 2 Acetyl CoA + 2 CO2 molecules. Lysine is: A. Solely a ketogenic amino acid B. Solely a glucogenic amino acid C. Both a ketogenic and a glucogenic amino acid D. Neither a ketogenic nor a glucogenic amino acid E. It is impossible to tell from this information

Answer 56

Solely a ketogenic amino acid

Question 57

The two nitrogen atoms in urea are derived from __________ and \_\_\_\_\_\_\_\_.

Answer 57

Ammonia and aspartate

Question 58

Nitrogen levels in the blood increase in the early stages of fasting because:

Answer 58

Amino acids become a major source for glucose production

Question 59

Carbamoyl phosphate synthetase catalyzes synthesis of carbamoyl phosphate from NH3, ATP and: A. Arginine B. Bicarbonate C. Urea D. Malonyl CoA E. Citrate

Answer 59

Bicarbonate

Question 60

Which two compounds along with 2 ATP are used to make carbamoyl phosphate?

Answer 60

Bicarbonate and ammonia